The protein involucrin is a precursor of the cross-linked envelope that forms late in the terminal differentiation of the human epidermal keratinocyte. This envelope is the most resistant structure in the skin and is the ultimate barrier between the body and the external environment. Involucrin synthesis begins when the keratinocyte reaches a position about halfway through the spinous layer of the epidermis, since that is the point at which the mRNA appears in the cytoplasm. Disorders in terminal differentiation occurring in neoplastic and pre-neoplastic conditions lead to abnormal patterns of involucrin synthesis that can be recognized by immunohistological methods. With a view to understanding how the synthesis of involucrin is regulated, we have isolated cDNA and genomic clones encoding this protein. We have obtained enough sequence data to establish that involucrin is a unique protein whose structure has probably never before been encountered. It possesses a repeating sequence of 10 amino acid residues. This sequence makes the protein a preferential substrate for cross-linking by the keratinocyte transglutaminase that forms the cell envelope. It is obvious from the repeating structure that involucrin evolved through a series of gene duplications that took place either in primates or their near ancestors. We have been able to prepare tryptic fragments of the protein and in one case have been able to map the fragment by comparing its amino acid sequence with the DNA sequence of genomic and cDNA clones. We hope to establish the entire sequence of the cDNA, and then begin to study the regulation of expression of the gene. (M)